The present invention relates to a method and apparatus for treating tows or ropes which consist of filamentary material. More particularly, the invention relates to improvements in banding of filamentary material which can be used as a filler in filter mouthpieces of cigarettes, cigars or cigarillos. Still more particularly, the invention relates to improvements in banding of a running tow or rope of filamentary material.
A tow of filamentary filter material (e.g., acetate fibers) must be treated in a filter rod making machine prior to conversion into the filler of a continuous filter rod. The filter rod is obtained by draping a web of wrapper material around the rod-like filler. Reference may be had to commonly owned U.S. Pat. No. 3,971,695 granted July 27, 1976 to Hans-Jurgen Block. This patent describes the conversion of a tow which is withdrawn from a bale into a rod-like filler. Such conversion involves spreading the tow to form a wide layer of substantially parallel filaments, spraying the layer with atomized plasticizer while the tow is stretched in the space between two pairs of rolls which rotate at different speeds, and causing the layer to pass through a so-called gathering horn which converts the layer into a rod. The plasticizer (e.g., triacetin) causes portions of neighboring filaments to adhere to each other so as to form a maze of intricate paths for the flow of tobacco smoke toward the smokers's mouth. In the next step, the continuous rod-like filler is draped into a web of cigarette paper, imitation cork or other suitable wrapping material, and the resulting continuous filter rod is subdivided into sections of desired length. For example, the filter rod can be subdivided into sections of six times unit length which are thereupon conveyed into the magazine of a filter tipping machine, e.g., a machine known as MAX manufactured by the assignee of the present application.
The conversion of a tow into a wide layer wherein the filaments are parallel or nearly parallel to each other can be effected by mechanical means. It is also known to resort to pneumatic banding devices, for example, to devices of the type disclosed in U.S. Pat. No. 3,345,697 granted Oct. 10, 1967 to Aspy. Two pneumatic banding devices are shown, at 21 and 24 in FIG. 1, in the aforementioned commonly owned U.S. Pat. No. 3,971,695 to Block. A drawback of presently known banding devices, including the pneumatic banding device of Aspy, is that they cannot convert a tow (whose width varies in random fashion) into a layer of constant or nearly constant width and that they cannot insure uniform distribution of filaments in the layer. Though the filaments of the tow are readily movable with respect to each other, presently known pneumatic banding devices are incapable of reliably converting the tow into a layer of uniform thickness, width and density because the accumulations of filaments in certain (thicker) strip-shaped portions of the running tow are not spread apart in the same way as the thinner strip-shaped portions of the tow. Streams of air which pass through the tow in a conventional pneumatic banding device flatten and increase the width of the tow; however, their equalizing or homogenizing action (as considered in a direction transversely of the direction of transport of the tow) is far from uniform. It must be borne in mind that the filaments of the tow which is withdrawn from a bale are crimped and are intimately interlaced with each other. The streams of air which are used in conventional banding devices are capable of widening and equalizing relatively thin portions but cannot achieve the same degree of equalizing action upon thicker strands of the tow. The patent to Aspy discloses a relatively wide box through which the tow advances and wherein a transversely extending wide nozzle discharges compressed air against one side of the tow. The stream of compressed air achieves a desirable widening of the tow and changes the orientation of filaments; however, its banding action is not entirely satisfactory, especially when the width of the tow varies within a wide range, i.e., when dense accumulations of filaments alternate with thinner portions which contain a lesser number of filaments.